Third son went down hard (4)
I believe the answer is:
seth
'third son' is the definition.
'seth' can be an answer for 'son' (I have seen 'son of Adam' mean 'seth' so perhaps 'son' could also mean 'seth'). I am not certain of the 'third' bit.
'went down hard' is the wordplay.
'went down' becomes 'set' (I can't explain this - if you can you should believe this answer much more).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'set'+'h'='SETH'
Can you help me to learn more?
(Other definitions for seth that I've seen before include "Early human" , "Vikram --, Indian author" , "Indian writer" , "Biblical character" , "One of the first names recorded" .)